Rechargeable alkaline manganese cell with cathode consistency compensation

ABSTRACT

In an improved rechargeable alkaline manganese cell that has a manganese dioxide cathode comprising pellets formed by pressing a cathode powder blend comprising a hygroscopic additive for increasing cumulative capacity, the sticky consistency of the pellets, which is un-desirable for continuous automated production is compensated for by the addition of up to 0.5% of a hydrophobic binder. This small amount leaves the cell performance substantially unimpaired, but provides the desired consistency for large-scale production. Further disclosed is an improved charge methodology for a rechargeable alkaline manganese cell wherein the charge current is pulsed at a voltage in excess of 1.65 V and the no-load cell voltage response is monitored at predetermined intervals. No charge current pulse is permitted to pass through the cell if the no-load voltage exceeds a threshold value. This results in increased utilization of the capacity of the cell while reducing the likelihood of damage to the cell due to overcharging.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/980,175, filed Nov. 4, 2004, which claims the benefit of CanadianAppln. No. 2,486,488, filed Nov. 1, 2004, which claims the benefit ofU.S. Patent Appln. No. 60/537,900, filed Jan. 22, 2004, the contents ofboth incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to rechargeable alkaline batteries. Specifically,the invention relates to cathode formulations of such batteries thatcomprise a hydrophobic cathode additive for affecting the consistency ofpressed cathode pellets to make them more amenable to continuousproduction. The invention also relates to an improved charging methodfor use with rechargeable alkaline manganese cells, particularly thecells of the present invention.

BACKGROUND OF THE INVENTION

Alkaline battery technology has been used since the 1970′s to provideinexpensive, long-lasting portable power sources for a variety ofelectrical applications. Disposable alkaline batteries, or primary cellsare the most common example. However, due to recent technicaladvancements, re-chargeable alkaline batteries, or secondary cells, haverecently become available. These batteries are significantly lessexpensive for the end-user and are also more environmentally benign. Inorder to avoid gas production during recharging that could lead todangerous internal overpressure and cell leakage, the chemistry ofrechargeable alkaline batteries is significantly different from primaryalkaline batteries. In order that rechargeable alkaline batteries areindistinguishable from primary alkaline batteries from a consumer'spoint of view, improvements need to be made to the cathode that resultin improved electrical performance and that allow the batteries to beproduced on conventional large-scale automated production equipment.

The cathode composition of primary alkaline batteries often comprises abinder that has the main task of increasing the flexural strength of thepressed cathode. Without the addition of sufficient binder (typicallyabout 1.0-2.0% by weight) excessive pellet breakage occurs duringautomated production. The addition of a binder has an adverse impact oncell performance; since binders are electrochemically inactive, thepresence of the binder reduces the quantity of active cathodecomponents, such as manganese dioxide, that are available to participatein electrochemical reactions. Also, binders are typicallynon-conductive. The addition of a binder is therefore tolerated inprimary alkaline batteries only because of its necessity for economiclarge-scale battery production.

There are many suitable binder materials. One such binder material ispolyethylene powder, particularly the polyethylene powder manufacturedunder the trade name Coathylene® by the Swiss firm Herberts PolymerPowders SA (“Herberts”). In July, 1999, Herberts published a studyentitled: “Coathylene® in Dry Cell Batteries”, and in November 1998,another publication entitled: “Precipitated LDPE fine powders as bindersin the manufacture of dry cell batteries”. These publications describeseveral advantages of Coathylene® powder when used in the cathodecomposition of a primary alkaline battery, such as: increased cathodestrength, decreased mechanical friction, no significant decrease inconductivity, etc. These advantages are present when the binder materialis added in a concentration of at least 1.0% and preferably 1.5 to 2.0%relative to the cathode mass. Lower concentration is not effective inachieving the aforementioned advantages.

Recent improvements in the chemistry of rechargeable alkaline batterycathode formulations have resulted in a desirable increase in cumulativedischarge capacity, cycle life, and discharge current.

One example of an improved cathode formulation is disclosed in Europeanpatent EP0617845 B1, by Taucher, et al. Taucher discloses a rechargeablealkaline cell with barium compounds such as BaO, Ba(OH)₂*8H₂0 and BaSO₄added to the cathode mix in the range of 3-25%. These barium compoundsprovide improved cumulative capacity, but also improve the flexuralstrength of the cathode, obviating the need for the addition of abinder. The addition of a binder to these cells is undesirable, sincethe presence of the binder reduces cell performance.

Another example of an improved cathode formulation is disclosed in U.S.Pat. No. 6,361,899, by Daniel-Ivad, et al., which is hereby incorporatedby reference for jurisdictions that permit this method. Daniel-Ivaddiscloses a rechargeable alkaline cell in which the cathode includeshygroscopic additive compounds comprising oxides, hydroxides, orhydrates of barium or strontium. These hygroscopic additives desirablyincrease the performance of the cell, as indicated by increases in thecumulative discharge capacity and cycle life of the cell.

While both of the foregoing references disclose the use of certainbarium compounds to improve cumulative cell capacity of rechargeablealkaline cells, neither reference contemplates continuous cellproduction nor addresses any of the issues that arise in a continuousproduction environment. In fact, when barium compounds such as BaO orBa(OH)₂*8H₂0 are used as additives, the formed pellets exhibit a“sticky” consistency that impedes continuous processing. As a result,the improved cathode formulations cannot be used in a continuousproduction environment, making batteries with these formulations tooexpensive for the end-user. Although binders are used to increaseflexural strength of the cathode as an aid in continuous pelletprocessing, binders are not normally selected to modify the consistencyof the cathode pellet. In fact, “stickiness” is not a problem forprimary alkaline batteries, as the cathode pellets in these cells do notcontain hygroscopic additives.

Accordingly, there is still a need for an improved rechargeable alkalinebattery cathode composition that results in increased batteryperformance while permitting manufacturing in a continuous productionenvironment.

Rechargeable alkaline cells are prone to cell failures when overchargingtakes place. Overcharge results in damage to cell components and willcause increased internal gassing, which in turn may eventually cause thecell to fail from electrolyte leakage due to overpressure. The criticalvoltage limit above which damage can occur is reported at 1.68V in theliterature (D. Linden, Handbook of Batteries, 2^(nd) Edition, Mc-GrawHill, New York, 1995). Typically, commercial chargers are voltagelimited at about 1.65V, which is the generally accepted safe value forrechargeable alkaline cells. However, a portion of the cell's capacityis not re-charged, which results in a loss of available performance. Theloss of available performance is especially significant when inertmaterials such as additives or binders are added to the cathode, as thepresence of these materials reduces the amount of active cathodecomponent available for electrochemical reaction.

The need therefore exists for an improved charging method forrechargeable alkaline cells, particularly cells containing additives andbinders.

SUMMARY OF THE INVENTION

In one aspect of the invention, there is provided a rechargeablealkaline manganese cell comprising a cathode pellet formed from ahomogeneous blend of: a cathode powder comprising manganese dioxide; ahygroscopic additive powder comprising oxides, hydroxides, or hydratesof barium or strontium; and, a hydrophobic binder for alteringconsistency of the pellet, the hydrophobic binder in an amount of lessthan 0.5% by weight of the cathode.

The present invention advantageously provides a cathode with anacceptable consistency when pressed and formed into a cathode pellet,without noticeably decreasing the electrical performance of the cellsmade using the pellet.

It has been discovered that a small amount of a hydrophobic bindermaterial can compensate for the aforementioned consistency problem incathode pellets of rechargeable alkaline manganese cells containinghygroscopic additives. The hydrophobic binder is preferably added in anamount greater than zero and less than 0.5% by weight of the cathode,more preferably in an amount of from 0.001% to 0.4% by weight of thecathode, even more preferably in an amount of from 0.01% to 0.25% byweight of the cathode, still more preferably in an amount of from about0.1% to about 0.2% by weight of the cathode. This amount is normally toosmall for performing a binder function as is conventionally understoodin the art.

Surprisingly, the addition of a hydrophobic binder in this amount doesnot appreciably decrease the electrical performance of the cells(cumulative cell capacity, cycle life, and discharge current) and, insome cases, may even result in a slight performance increase.

The hydrophobic binder may be a polyethylene material (for example,Coathylene®), a polytetrafluoroethylene material (for example, Teflon®),or a metal salt of a fatty acid (for example, calcium stearate,magnesium stearate or zinc stearate). Preferably, the hydrophobic binderis provided in a powdered form with a particle size less than 75 μm,preferably between 10-20 μm. More preferably, the hydrophobic binder isa powdered polyethylene material comprising Coathylene® powder. Evenmore preferably, the hydrophobic binder is a powdered materialcomprising calcium stearate.

In another aspect of the invention, there is provided a method ofcharging a rechargeable alkaline manganese cell comprising the steps of:applying a first charge voltage in excess of 1.65 V to the cell at acharge frequency, resulting in a series of current pulses; applying nocharge voltage to the cell for a pre-determined time interval followingeach current pulse; measuring a no-load voltage of the cell at ameasurement frequency offset from the charge frequency by thepre-determined time interval; comparing the no-load voltage to the firstcharge voltage; and, when the no-load voltage exceeds the first chargevoltage, skipping the next subsequent current pulse.

The charge methodology of the present invention advantageously resultsin increased charging efficiency and cumulative cell capacity ofrechargeable alkaline cells in a safe and automatable manner.Approximately a 15% increase in cell capacity may be obtained. A currentpulse may be permitted to pass through the cell only if the no-loadvoltage does not exceed the first charge voltage to prevent damage tothe cell through overcharging. The first charge voltage may be 1.75 V.The current pulse may have a duration of 14.5 seconds. Thepre-determined time interval may be 0.5 seconds.

In another embodiment, the method may further comprise: counting thenumber of skipped current pulses; when the number of skipped pulses isgreater than or equal to a first pulse-skip value, comparing the no-loadvoltage to a second charge voltage; skipping each subsequent currentpulse until the no-load voltage is less than the second charge voltage;followed by, when the no-load voltage exceeds the second charge voltage,skipping the next subsequent current pulse. The second charge voltagemay be 1.70 V and the first pulse-skip value may be 6 current pulses.

The method may yet further comprise: when the number of skipped currentpulses is greater than or equal to a second pulse-skip value, comparingthe no-load voltage to a third charge voltage; skipping each subsequentpulse until the no-load voltage is less than the third charge voltage;followed by, when the no-load voltage exceeds the third charge voltage,skipping the next subsequent current pulse. The third charge voltage maybe 1.65 V and the second pulse-skip value may be 24 current pulses.

An automated charger may also be provided for practicing the methodaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Production Efficiency

Pellets sized for AA rechargeable alkaline manganese dioxide cells wereproduced substantially as described in Example 1 of previously citedU.S. Pat. No. 6,361,899. The hygroscopic additive barium hydrate(Ba(OH)2*8H2O) was used in making the cathode blend instead of bariumsulphate, as described. Cathode pellets were made by pure pressing ofthe mixed components (i.e. no binder material was used) and theconsistency of the pellets was sticky.

In pellets made according to the present invention, the same procedurewas followed as for control cells, with the addition of a hydrophobicbinder comprising the polyethylene powder Coathylene® type HA1681 in anamount of about 0.1%, 0.15% and 0.2% by weight relative to the cathodemass. The consistency of the cathode pellets obtained from these mixesdid not exhibit the sticky consistency of the control cells.

A pellet consolidation machine (provided by Hibar Sytems Ltd. ofRichmond Hill, Canada) was used for continuous production trials withboth types of pellets at cell production speeds of 200 cells/min. Threepellets were used in each cell, resulting in a pellet feed rate of 600pellets/min. The control cathode pellets with a sticky consistencyresulted in blockages inside the pellet infeed tracks. Blockages causedstoppage of the automated equipment until an operator was able to clearthe blockage to resume production. This caused a significant reductionin production throughput. The production speed of 200 cells/min couldnot be maintained continuously. The loss in production efficiencydepends on the number of blockages and the time it takes the operator toclear each blockage. At an average rate of 10 blockages per hour, and anaverage time of 60 seconds to clear and resume production for eachblockage, a loss in production efficiency of approximately 17% wasobserved with the control pellets. No corresponding pellet blockages (orresultant efficiency loss) was observed for the pellets made accordingto the invention during a timed continuous run of 2 hours. A comparisonof the production efficiency for the control pellets (made according tothe prior art) and pellets with a hydrophobic binder made according tothe present invention is provided in Table 1.

TABLE 1 Production Efficiency Comparison Control Invention Coathylene ®HA1681 0.00% 0.10% 0.15% 0.20% No. of blockages per hour 10 0 0 0 Lossin efficiency   17%   0%   0%   0%

Table 1 above illustrates that, surprisingly, only small amounts of thehydrophobic binder are required in the cathode blend to overcome theproblem of pellet blockages and efficiency loss during continuousproduction.

Discharge Capacity

From an electrochemical point-of-view, the addition of a hydrophobicbinder to the cathode is counter-intuitive. The addition of an‘inactive’ material (polyethylene is does not participate in theelectrochemical reaction and is non-conductive) reduces the amount of‘active’ material available and also the conductivity of the cathodemix. It is worth noting that polyethylene powder has a specific densityof about 0.916 g/mL vs. 4.29 g/mL for electrolytic manganese dioxide;hence, the addition of 1% polyethylene powder by weight replaces 4.7% ofthe pellet volume with a non-conductive material. As a result, anincreasing amount of polyethylene powder will decrease the availabledischarge capacity in the cell.

For cells according to the present invention with the polyethylenepowder Coathylene® used as the hydrophobic binder, the effect ofpolyethylene powder addition on discharge capacity loss is illustratedin Table 2. The loss of theoretical capacity is calculated for pelletswith increasing Coathylene® content on the basis that Coathylene® isreplacing electrolytic manganese dioxide in the cathode mix. Theremaining ‘free air’ or porosity in the pellets is assumed constant;this causes the apparent density of the pellets to decrease withincreasing polyethylene content. AA cells were made with the pelletsfrom these cathode mixes and discharge capacity was measured through a10-ohm resistive load to a cut-off voltage of 0.9V. For the very highCoathylene® addition levels of 1.5% and 2.0% no cells were made, asthese high levels already exhibited a theoretical capacity loss that wastoo high for practical consideration.

TABLE 2 Discharge Capacity As A Function of Polyethylene ContentCoathylene ® HA1681 0.0% 0.1%   0.2%   0.4%   1.0% 1.5% 2.0% Loss oftheoretical capacity 0.0% −0.4%   −0.8% −1.7% −4.2% −6.4% −8.4% RelativeInitial discharge   0%   0%    +3%   −4%   −7% Not Not capacity on10-ohm load to tested tested 0.9 V

As can be seen form Table 2, the cells with 0.1% and 0.2% Coathylene®performed essentially similar to the control cells without anyCoathylene® addition. At 0.4% and 1.0% addition, the actual loss washigher than the theoretical loss, which can most likely be attributed tothe addition of the non-conductive binder to the cathode pellets. It issurprising to note that the addition of 0.1% to 0.2% polyethylene, alevel that is considered in the literature to provide no binderfunctions, performed at least equal to or slightly better than thecontrol.

Cell Re-Charging

In order to charge more of the available capacity of the cell, a voltagein excess of the accepted maximum of 1.65 V is applied. To avoid damageto the cell due to overcharging, the voltage is applied in discretepulses at a certain charge frequency. Depending on the duration of thepulse, the pulses may alternatively more closely resemble astep-function. The no-load cell voltage response is measured at ameasurement frequency that is normally equal to the charge frequency,but offset therefrom by a time interval. The time interval is selectedso that a period of no-load occurs after a charge pulse that is suitableto allow a steady-state no-load voltage measurement to be taken. Whenthe no-load voltage measurement is greater than or equal to the chargingvoltage threshold, the next subsequent charge pulse is skipped. When adesired pre-determined pulse to no-pulse ratio is obtained, the chargingvoltage may be adjusted. The charge frequency, pulse duration, or offsetinterval may also be adjusted.

In a first embodiment, a charge current of 300-350 mA is applied. Thecharge frequency is one pulse every 15 seconds having a duration of 14.5seconds, or 4 pulses/minute. The measurement frequency is equal to thecharge frequency (4 measurements/minute), but offset by a no-load timeinterval of 0.5 seconds from each charge pulse. In a first chargingphase, the first threshold voltage is 1.75 V. When the no-load voltageis equal to or greater than the first threshold voltage, the nextsubsequent pulse in the series of pulses defined by the chargingfrequency is skipped. The next measurement is taken at the usual time,as if the pulse had not been skipped. When the ratio of pulses toskipped pulses is 1:6, the first charging phase ends. Normally, theratio of 1:6 is attained when six skipped pulses are countedconsecutively.

The charging method may utilize any number of discrete charging phases.In a second embodiment there are two charging phases and in a thirdembodiment there are three charging phases.

In the second embodiment, the first charging phase is as described abovefor the first embodiment and the second charging phase beginsimmediately after the first charging phase ends. At the beginning of thesecond charging phase, pulses are skipped until the no-load voltage isless than or equal to the second threshold voltage of 1.70 V. Pulses of300-350 mA are then applied at the second threshold voltage. When themeasured no-load voltage exceeds the second threshold voltage, the nextsubsequent pulse in the charge cycle is skipped. When 24 consecutiveskipped pulses are counted, the second charging phase ends.

In the third embodiment, the first and second charging phases are asdescribed above for the first and second embodiments. The third chargingphase begins immediately after the second charging phase ends. At thebeginning of the third charging phase, pulses are skipped until theno-load voltage is less than or equal to the third threshold voltage of1.65 V. Pulses of 300-350 mA are then applied at the third thresholdvoltage. When the measured no-load voltage exceeds the second thresholdvoltage, the next subsequent pulse in the charge cycle is skipped. Pulseskipping is permitted to continue indefinitely until the batteries areremoved from the charging device used to apply the method or until apre-set time limit is reached (for example, 24 hours).

An automated charger can be programmed to apply the charging method. Theautomated charger preferably utilizes a micro-controller forimplementing the method. The values for the various parameters of themethod may be fixed or may be user adjustable. Parameters that have beenfound suitable are as follows:

-   1. initial charge current per cell of 300 to 350 mA-   2. charging and monitoring frequency of 4 per minute-   3. no-load time interval for voltage response measurements of 0.5    seconds after each pulse-   4. charge phase 1 at a no-load voltage limit of 1.75V    -   a) apply charge current for 14.5 seconds duration until no-load        voltage response at the 0.5 second no-load measurement point        exceeds 1.75V    -   b) start skipping charge pulses if no-load voltage response is        above 1.75V    -   c) allow charge pulse if no-load voltage response is below 1.75V    -   d) continue pulse skipping until pulse to no pulse ratio is 1 to        6 (6 skipped pulses)    -   e) commence charge phase 2-   5. charge phase 2 at a no-load voltage limit of 1.70V    -   a) rest at no-load until no-load voltage response is below 1.70V    -   b) allow charge pulse if no-load voltage response is below        1.70V, skip pulse if above    -   c) continue pulse skipping until pulse to no pulse ratio is 1 to        24 (24 skipped pulses)    -   d) commence charge phase 3-   6. charge phase 3 at a no-load voltage limit of 1.65V    -   a) rest at no-load until no-load voltage response is below 1.65V    -   b) allow charge pulse if no-load voltage response is below        1.65V, skip pulse if above    -   c) continue pulse skipping until a total charge time of 12 or 24        hours is reached

Although the charging method may be employed with any rechargeablealkaline manganese cell, the method is particularly advantageouslyapplied to cells of the present invention. Due to the presence of thehydrophobic binder, which is electrochemically inactive andnon-conductive, the capacity of the cells of the present invention isdiminished at the outset as compared with the control. The presentcharging method allows more of the available capacity of the cells to beutilized, making the cells of the present invention indistinguishable interms of performance from prior art cells charged with prior artcharging methods.

Charging Method Comparison

The control and the cells according to the present invention weresubjected to 25 discharge-charge cycles. Each cycle consisted ofdischarge using a 10 ohm resistive load to discharge the cells to acut-off voltage of 0.9V followed by charging according to either thestandard charge method or the new charge method. The standard chargemethod consisted of charging for 12 hours to a voltage limit of 1.65 Vwith an initial charge current of approx. 400 mA applied continuously.As the cell voltage increases, the actual charge current decreases andtapers off to almost zero as the 1.65 V limit is reached, hence thismethod is also referred to as ‘taper’ charge. The new charge method wasaccording to the third embodiment, as described above, also conductedfor 12 hours.

Table 3 shows the capacity of the 25^(th) cycle and the cumulativecapacity over 25 cycles for all cells indicating the percentage changesof the different charge methods. The term cumulative capacity means thesum of all individual discharge capacities over the tested number ofcycles. The given data represent the average of 4 cells per test in eachgroup.

TABLE 3 Charging Method Comparison with Cycle Test at 10 Ohm LoadCumulative 25 25th Cycle [Ah] Performance Cycles [Ah] Performance Std.New Increase for Std. New Increase for Cell type HA1681 charge chargeNew charge charge charge New charge Control 0.00% 0.94 1.00 +6.38% 26.1031.15 +19.35% #1 0.10% 0.86 0.92 +6.98% 26.40 31.00 +17.42%

From Table 3 it can be seen that for both types of cells the new chargemethod provides much improved cumulative performance over 25 cycles.Approximately 90% of the theoretical maximum charge capacity is reachedwith cells charged using the new charging method, as compared with atypical value of 75% for the standard charging method. The individualcycle capacity in the 25^(th) cycle is increased as well, indicating alower capacity fade per cycle. The cumulative capacity of the cellsaccording to the present invention charged with the new charging methodsurprisingly exceeds the cumulative capacity of the control cellscharged using the prior art charging method, despite having less activecathode material due to the presence of the hydrophobic binder. The cellperformance observed by the end-user is therefore actually superior forthe cells of the present invention. Although the new charging method isparticularly advantageously applied to cells of the present invention,it may also be advantageously applied to the control cells.

Cumulative Cell Performance

The addition of hydrophobic binder theoretically decreases thecumulative capacity and capacity of the 25^(th) cycle of the cells. Theeffect of hydrophobic binder addition was investigated using controlcells and cells according to the present invention. Twenty-fivedischarge-charge cycles were conducted as described above. The dischargewas performed using both a 10 ohm resistive load to a cut-off voltage of0.9 V and a 3.9 ohm resistive load to a cut-off voltage of 0.8 V. Thelower resistive load resulted in a higher discharge current. The cellswere then recharged using the new charge method for a period of 12hours. The results of these tests are compiled in Tables 4 and 5. Theterm cumulative capacity means the sum of all individual dischargecapacities over the tested number of cycles. The data represents theaverage of 4 cells in each group.

TABLE 4 Comparison of Binder Amounts with Cycle Test at 10 Ohm LoadCumulative Cell Coathylene ® 25th Cycle 25 Cycles type HA1681 [Ah]Change [Ah] Change Control 0.00% 1.00 0.00% 31.15 0.00% #1 0.10% 0.92−8.00% 31.00 −0.48% #2 0.20% 0.87 −13.00% 30.86 −0.93%

The presence of the hydrophobic polymer powder had only a negligibleimpact on cumulative cell capacity and only slightly decreased thecapacity of the 25^(th) cycle. The decrease was more remarkable forcells #2 with a higher amount of Coathylene® HA1681.

TABLE 5 Comparison of Binder Amounts with Cycle Test at 3.9 Ohm LoadCell Coathylene ® 25th Cycle Cumulative type HA1681 [Ah] Change 25Cycles [Ah] Change Control 0.00% 0.83 0.00% 26.85 0.00% #1 0.10% 0.84+1.20% 26.44 −1.53% #2 0.20% 0.86 +3.61% 25.45 −5.21%

For a higher discharge current the presence of the hydrophobic polymerpowder slightly increased the capacity in the 25^(th) cycle and onlyslightly decreased the cumulative capacity over 25 cycles. Again, thedecrease was more remarkable for cells #2 for cells #2 with a higheramount of Coathylene® HA1681. The tendency demonstrated by these tests(i.e. that performance decreases with increased presence of thehydrophobic polymer powder) indicated that higher levels of polyethylenepowder additive would be detrimental to cumulative cycle performance,although with small amounts the decrease in performance was stillacceptable.

Other Hydrophobic Binders

Another suitable class of hydrophobic binders comprises a metal salt ofa fatty acid. Examples of suitable fatty acids are stearic acid,margaric acid and palmitic acid. One particularly suitable fatty acid isstearic acid. Examples of metal salts of stearic acid are calciumstearate, magnesium stearate and zinc stearate. The hydrophobic bindercalcium stearate has a chemical formula of Ca[CH₃(CH₂)₁₆CO₂]₂. Thishydrophobic binder is particularly preferable, as it provides thehydrophobic characteristic from the —(CH)₂— fatty acid chain and alsoprovides a calcium compound (a possible second hygroscopic additive inaddition to barium and strontium).

To demonstrate the suitability of calcium stearate, test cells #3 weremade with calcium stearate in an amount of 0.4% by weight of the cathodeas the hydrophobic binder. The pellets produced did not exhibit anysticky consistency and could be continuously processed without pelletblockages or efficiency loss on automated equipment as previouslydescribed.

In order to study the effect of this hydrophobic binder on cumulativecell capacity, test cells #3 were subjected to 25 discharge-chargecycles using a 10 ohm resistive load to discharge the cells to a cut-offvoltage of 0.9V and applying the new charging methodology as describedabove for 12 hours. The results for this test are compiled in Table 6.The term cumulative capacity means the sum of all individual dischargecapacities over the tested number of cycles. The data represents theaverage of 4 cells in each group.

TABLE 6 Calcium Stearate Binder Cycle Test at 10 Ohm Load 25th CycleCumulative Cell type CaStearate [Ah] Change 25 Cycles [Ah] ChangeControl 0.00% 1.00 0.00% 31.15 0.00% #3 0.40% 1.07 +7.00% 32.3 +3.69%

The hydrophobic calcium stearate powder increased the cumulativecapacity and capacity of the 25^(th) cycle by 3.69% and 7.00%,respectively. The addition of 0.4% calcium sterate as hydrophobic binderdid not decrease cumulative or 25^(th) cycle capacity, but surprisinglyprovided improved capacity. This is likely due to the presence of thecalcium compound, which serves as a source for Ca ions and acts as anadditional additive in the cathode mix.

In keeping with this concept, different fatty acids and different metalsalt compounds, are deemed to be suitable. The fatty acids should beselected to have sufficient —(CH)2-chains to provide sufficienthydrophobicity; typically 12 to 18—(CH)2-chains in the fatty acidformula are suitable candidates. Further additives that could solve the‘stickiness’ problem would be from the group of hydrophobic polymerpowders, such as polytetrafluoroethylene (Teflon®) and others. However,Teflon® powders are more costly than polyethylene and therefore lessdesirable.

From the foregoing, it will be seen that this invention is one welladapted to attain all the ends and objects hereinabove set forthtogether with other advantages which are obvious and which are inherentto the structure.

It will be understood that certain features and sub-combinations are ofutility and may be employed without reference to other features andsub-combinations. This is contemplated by and is within the scope of theclaims.

Since many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth is to be interpreted as illustrative and not in alimiting sense. Variations of the foregoing embodiments will be evidentto a person of ordinary skill and are intended by the inventor to beencompassed by the following claims.

1. A rechargeable closed alkaline manganese cell comprising: a. anaqueous electrolyte; and, b. a cathode pellet formed from a homogeneousblend of: i. a cathode powder comprising manganese dioxide; ii. anadditive powder comprising fully hydrated strontium; and, iii. ahydrophobic binder for altering consistency of the pellet, thehydrophobic binder comprising a fluorinated polymer in an amount ofgreater than zero and less than 0.5% by total weight of the cathodepellet.
 2. The cell according to claim 1, wherein the fluorinatedpolymer comprises polytetrafluoroethylene.
 3. The cell according toclaim 1, wherein the hydrophobic binder is present in an amount of from0.01% to 0.4% by weight of the cathode.
 4. The cell according to claim3, wherein the hydrophobic binder is present in an amount of from 0.01%to 0.25% by weight of the cathode.
 5. The cell according to claim 4,wherein the hydrophobic binder is present in an amount of from 0.1% to0.2% by weight of the cathode.
 6. The cell according to claim 1, whereinthe hydrophobic binder is provided in powdered form with a particle sizeof less than 75 μm.
 7. The cell according to claim 6, wherein thehydrophobic binder has a particle size of from 10 to 20 μm.
 8. The cellaccording to claim 1, wherein the hydrophobic binder consistsessentially of polytetrafluoroethylene.
 9. The cell according to claim8, wherein the hydrophobic binder is present in an amount of from 0.01%to 0.4% by weight of the cathode.
 10. The cell according to claim 9,wherein the hydrophobic binder is present in an amount of from 0.01% to0.25% by weight of the cathode.
 11. The cell according to claim 10,wherein the hydrophobic binder is present in an amount of from 0.1% to0.2% by weight of the cathode.
 12. The cell according to claim 8,wherein the hydrophobic binder is provided in powdered form with aparticle size of less than 75 μm.
 13. The cell according to claim 12,wherein the hydrophobic binder has a particle size of from 10 to 20 μm.